Smooth coffee swallowed by disheartened fisherman (7)
I believe the answer is:
flatten
'smooth' is the definition.
(I've seen this before)
'coffee swallowed by disheartened fisherman' is the wordplay.
'coffee' becomes 'latte' (type of coffee drink).
'swallowed by' means one lot of letters goes inside another.
'disheartened' means to remove the middle letters (the middle or heart is removed).
'fisherman' with its middle removed is 'fn'.
'latte' going within 'fn' is 'FLATTEN'.
(Other definitions for flatten that I've seen before include "Bulldoze" , "'Crush, trample (7)'" , "Make smooth or level" , "Raze to the ground" , "Stop" .)